The invention relates to a method for cleaning objects in pressure vessel with liquefied gases.
A method of this kind for cleaning workpieces, especially metal pipes, containing organic residues such as oils and greases is known from patent application WO92/14558. In this application, liquefied gases such as carbon dioxide are used as the cleaning fluid. The cleaning fluid is conducted into a pressure vessel loaded with the workpieces and circulated therein by means of an impeller. After the cleaning process is complete, a portion of the fluid laden with organic residues is conducted out of the pressure vessel together with fresh cleaning fluid into another pressure vessel. The surface tension of the remainder of the fluid loaded with impurities is reduced by a turbine, causing the impurities to precipitate out. The cleaned workpieces are then removed from the empty pressure vessel, while additional workpieces can be cleaned in the second pressure vessel.
In addition, a device for cleaning smaller workpieces that uses liquefied gases is known from German patent document DE-42 30 385. In the German patent document, a drum is mounted rotatably and/or pivotably in a pressure-tight container and is connected to a drive device. The rotation of the drum sets both the cleaning fluid in the pressure-tight vessel and the objects present in the drum in turbulent motion, thus increasing the cleaning effect. Chemical solvents, mechanical scouring agents, and added insert gases can increase the cleaning effect. To permit quasi-continuous operation, at least one lock for loading and unloading the objects is provided on the pressure-tight vessel. These locks make it unnecessary to completely vent the pressure-tight vessel to load or remove the objects.
Liquid carbon dioxide at temperatures between 20xc2x0 C. and approximately 30xc2x0 C. is used in the method in the two patent documents mentioned above, with pressures corresponding to the vapor pressure values.
In addition, a method for cleaning textiles by means of liquefied or supercritical carbon dioxide is known from WO 94/01613. In this patent document, the carbon dioxide is conducted into a pressure vessel loaded with the textiles at temperatures between 20xc2x0 C. and 100xc2x0 C. and corresponding pressures between 60 and 350 bars. After the textiles have been washed, displacement rinsing is performed using a second fluid. The second fluid is again a compressed gas such as air or nitrogen.
A method for removing impurities from a substrate is known from WO 90/06189. In this patent document, a substrate is brought into contact by means of a compressed gas with fluid-like density at critical or supercritical pressure, with the phase of the gas being shifted between the liquid state and the supercritical state by varying the temperature of the gas in a series of stages between supercritical temperatures and subcritical temperatures. This multistage temperature variation is discontinued by a change in the cohesion energy content of the gas in the dense phase. An effort is therefore made to adjust the solvency of the compressed gas relative to an impurity in an effective manner. Mechanical interactions between the compressed gas and the impurities are not taken into account.
These known cleaning methods utilize the cleaning ability of compressed fluids, which increases drastically in the supercritical range. Supercritical carbon dioxide with a temperature above 31xc2x0 C. at pressures above 73.7 bars has a density comparable to the liquid phase and a good solvency that increases even further with an increase in temperature. Of course, the interactions of the fluid with the substance to be dissolved (impurities) are important for the success of the cleaning.
In the practical application, these cleaning methods suffer from an important disadvantage that lies in the use of high (supercritical) pressures, which in turn necessitates the use of expensive pressure vessels with high energy and equipment costs.
The goal of the present invention is therefore to develop a cleaning method using liquefied gases in which the same cleaning results can be achieved at lower cost.
This goal is achieved according to the invention by virtue of the fact that gas liquefied under pressure is conducted into the pressure vessel containing objects to be cleaned. The temperature of the liquefied gas is then (or even earlier) lowered below the critical temperature of the gas. Cleaning is conducted at least primarily below the critical temperature and below the critical temperature of the gas.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.